Galaxy

A Galaxy was a vast, gravitationally bound collection of stars, planets, gas, dust, stellar remnants and dark matter. They were some of the largest single structures in the Universe, and over 1 trillion Galaxies were known to exist.

Galaxies varied enormously in size, shape and behavior. Galaxies could contain millions, billions or even trillions of star systems and planets, which themselves could give rise to life. Galaxies often collided, sometimes ejecting huge numbers of stars into intergalactic space. Many Galaxies developed around huge masses of dark matter, which themselves developed around supermassive black holes. Many galaxies contained supermassive black holes in their cores.

The most famous Galaxy was the Skyriver Galaxy, which was only known by this name in order to distinguish it from other galaxies, and was almost universally referred to simply as The Galaxy. Almost all of recorded history, from the time of the Celestials through the age of the Republic all the way to the events of the Legacy Era, took place in this Galaxy.

Formation
Galaxies formed primarily through the action of supermassive black holes, which drew huge quantities of dark matter into their midst. The dark matter caused immense quantities of regular matter to surround the black hole, creating a vast cloud of material. The density of the gas triggered the formation of stars and planets. The vast clouds of material usually organized themselves to become spiral galaxies, which often eventually evolved into elliptical galaxies.

Irregular galaxy
Irregular galaxies were extremely common. Almost all small galaxies were irregular, often appearing similar to large nebulae. Most larger galaxies also began as irregular galaxies, but then evolved into spiral galaxies.

Spiral galaxy
These galaxies were medium size, and often formed from irregular galaxies that contained a large enough black hole to force the galactic material into a regular orbit. The Skyriver Galaxy and Gala Yun were both spiral galaxies.

Barred spiral galaxy
These were spiral galaxies with a thick, central bar of stars and surrounding material. These dense regions were usually caused by gravitational interaction with another galaxy.

Ring galaxy
These were spiral galaxies with very thick mid and outer rims but sparse cores. They were often formed when a small galaxy penetrated a large spiral galaxy.

Lenticular galaxy
These were large galaxies that exhibited traits of both spiral and elliptical galaxies, being in an evolutionary stage between both.

Elliptical galaxy
These were the largest and oldest galaxies, and had an elliptical shape. They were often formed through the merging of multiple smaller spiral galaxies.

Shell galaxy
These were elliptical galaxies with stars arranged into concentric shells. They formed through the absorption of a smaller galaxy by a large elliptical galaxy.

Diffuse galaxy
These were galaxies with extremely low densities. Often they were small galaxies stretched to immense sizes by gravitational interaction with other galaxies. They could also be galaxies with low hydrogen abundance, resulting in low star formation.

Mature galaxy
These were galaxies with little or not violent activity, normal levels of star formation and no interaction/collision with other galaxies. The Skyriver Galaxy was a mature galaxy.

Interacting galaxy
These were groups of two or more galaxies touching or colliding with one another.

Starburst galaxy
These were galaxies with high rates of star formation.

Super-luminous spiral galaxy
These were very large spiral galaxies with immense luminosity caused by very fast star formation.

Active galaxy
These galaxies' central black holes had active accretion disks, releasing great amounts of energy.

Radio galaxy
These were active galaxies that released huge amounts of radio waves, often making radio communication difficult or even impossible for societies living within.

Infrared galaxy
These were active galaxies that released huge amounts of infrared radiation, and relatively little visible light. Due to this they were physically invisible despite having a huge energy output.

Liners
These were active galaxies where the activity of the black hole triggered massive star formation.

Seyferts
These were active galaxies with immensely bright black hole accretion disks, but they were still stable enough for interstellar civilization to continue existing.

Quasars
These were active galaxies with black hole accretion disks so bright that they largely obscured the rest of the galaxy, making the galactic structure invisible from afar. Quasars brought severe disruption to interstellar civilizations living within the galaxy. Gala Yun, the home galaxy of the Yuuzhan Vong, was a Quasar.

Blazars
These were active galaxies with relativistic jets forming from their accretion disks. These jets were visible across the Universe.